This invention relates to a catalytic cracking process for treating hydrocarbon feedstocks of high metals content while minimizing or preventing losses in catalytic activity or selectivity due to metals deposition on the cracking catalyst. The invention further relates to a method for passivating metals deposited on cracking catalyst particles and for restoring a substantial portion of the activity and selectivity of cracking catalyst particles having metal contaminants on the surfaces thereof.
In the petroleum industry, a high boiling hydrocarbon feedstock is charged to a catalytic cracking unit, such as a fluidized catalytic cracking unit, so that, by contact with a moving bed of catalyst particles, the feedstock is converted to a more valuable hydrocarbon product of lower average molecular weight and lower average boiling point (e.g., gasoline). As is well-known, the feasibility of cracking a particular feedstock depends in large measure upon the concentration of nickel, vanadium, iron, copper, and other metals in the feedstock. These contaminant metals deposite upon the catalyst surfaces and cause a gradual decrease in the production of intended gasoline product and an increase in the production of undesired products such as coke and hydrogen. The higher the concentration of metals in the feedstock, the more rapid is the deposition of metals upon the catalyst surfaces. Consequently, the life of a catalyst for cracking purpoes varies inversely with the contaminant metals concentration in the feedstock, i.e., the larger the concentration of metals, the shorter the life of a catalyst for producing a minimum gasoline yield from a given hydrocarbon feedstock being catalytically cracked under a given set of conditions. Usually, the life of a cracking catalyst proves economically justified when the hydrocarbon feedstock contains less than 1 wppm of total metal contaminants (calculated as the sum of the vanadium, copper, iron, and nickel contaminants) and on occasion may prove justified when the feedstock contains less than about 4 wppm of total metal contaminants.
Several methods have been proposed to overcome the problems associated with metals deposition on cracking catalysts. One general method involves pretreating a hydrocarbon feedstock to reduce the metals concentration to an economically tolerable level for catalytic cracking feeds. One such pretreating method involves fractionating a hydrocarbon liquid to concentrate the metals therein into a heavy fraction and thereby produce a lighter, essentially metals-free fraction suitable as a cracking unit feedstock. In an alternative pretreatment method, the entire hydrocarbon liquid is subjected to catalytic demetallization, that is, the metals-containing hydrocarbon liquid is passed through a bed of catalytically active material that absorbs the metals and leaves a demetallized hydrocarbon product. Other methods for coping with the metals deposition problem involve removing the metal contaminants after they deposit on the catalyst or providing a catalyst, as described in U.S. Pat. No. 3,944,482, that remains highly active for hydrocarbon cracking reactions despite high concentrations of contaminant metals in the feedstock. Still other techniques are directed to introducing into the catalytic cracking unit one or more "passivating" agents that decrease the poisoning effects of the deposited metals. Methods illustrating this approach with manganese, bismuth, and antimony passivators are disclosed in U.S. Pat. Nos. 3,711,422 and 3,977,963.
At the present time, none of the above-described methods has found widespread use in the petroleum industry, largely because of the costs involved. Pretreating a feedstock for no other reason than to reduce the metals concentration requires an investment in capital equipment that seldom proves economically justified. Similarly, the added operational costs of metals-tolerant catalysts or compounds of bismuth and antimony often outweigh the advantages gained with such expensive materials. And although there are many disclosed processes for removing metals from cracking catalysts, such processes are very costly, requiring external reactivation facilities and expensive reactivation chemicals to restore, in the usual case, only a small portion of the original activity and selectivity of the catalyst.
Accordingly, it is an object of the invention to provide a process for maintaining the activity and selectivity of a cracking catalyst and thus prolong its useful life by utilizing inexpensive aluminum components to passify metal contaminants that deposit on the catalyst. It is a specific object of the invention to passivate metal contaminants with which cracking catalysts come in contact by blending organic aluminum compounds into a metals-containing feedstock entering the cracking unit. It is yet another object of the invention to modify the cyclic catalytic cracking process so that hydrocarbon feedstocks containing more than about 5 wppm of metal contaminants can be treated without unduly shortening the life of the cracking catalyst. It is still another object of the invention to reduce the rate at which catalyst particles containing metal contaminants are removed from cracking units, thereby also reducing the rate at which fresh catalyst particles must be added to the cracking unit. Other objects and advantages will become apparent to those skilled in the art from the following description of the invention.